In recent years, the necessity to decrease the degree of dependence on petroleum as an energy source has been pointed out from the viewpoint of reducing carbon dioxide emission and securing energy, for the purpose of which technologies have been actively developed. Among such technologies, hybrid vehicles and electric drive vehicles that use an electrical power source (battery) as a driving source have raised great expectations. In this description, hybrid vehicles and electric drive vehicles are collectively referred to as electric vehicles. Various attempts have been made in order to improve the use efficiency of electrical energy as a power source for electric vehicles.
Patent Literature 1 discloses a system in which a DC-DC converter 103 is arranged between a high voltage power source 101 and a drive inverter 104, as shown in FIG. 9. In the system of Patent Literature 1, a control device 109 transmits control signals to the DC-DC converter 103 and the drive inverter 104, on the basis of the rotational speed and the torque of a drive motor 110. Thus, the boost ratio in the DC-DC converter 103 and the PWM duty in the drive inverter 104 are sequentially changed. Further, the control device 109 transmits control signals also to a compressor inverter 107, thereby sequentially changing the PWM duty in the compressor inverter 107 so as to supply an appropriate AC voltage to a compressor motor 111.